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Solar System Lecture 2 Solar system Beibei Liu (刘倍⻉) Introduction to Astrophysics, 2019 What is planet? Definition of planet 1. It orbits the sun (central star) 2. It has sufficient mass to have its self-gravity to overcome rigid bodies force, so that in a hydrostatic equilibrium (near round and stable shape) 3. Its perturbations have cleared away other objects in the neighbourhood of its orbit. asteroid, comet, moon, pluto? Moon-Earth system: Tidal force Tidal locking (tidal synchronisation): spin period of the moon is equal to the orbital period of the Earth-Moon system (~28 days). Moon-Earth system: Tidal force Earth rotation is only 24 hours Earth slowly decreases its rotation, and moon’s orbital distance gradually increases Lunar and solar eclipse Blood moon, sun’s light refracted by earth’s atmosphere. Due to Rayleigh scattering, red color is easy to remain Solar system Terrestrial planets Gas giant planets ice giant planets Heat source of the planet 1. Gravitational contraction: release potential energy 2. Decay of radioactive isotope，such as potassium, uranium 3. Giant impacts, planetesimal accretion Melting and differentiation of planet interior Originally homogeneous material begin to segregate into layers of different chemical composition. Heavy elements sink into the centre. Interior of planets Relative core size Interior of planets Metallic hydrogen: high pressure, H2 dissociate into atoms and become electronic conducting. Magnetic field is generated in this layer. Earth interior layers Crust (rigid): granite and basalt Silicate mantle: flow and convection Fe-Ni core: T~4000-9000K Magnetic field is generated by electric currents due to large convective motion of molten metal in outer core and mantle Earth interior layers Tectonics motion Large scale movement of plates in Earth’s lithosphere. Tectonics motion Much of this motion causes earthquakes, builds mountains, rand, indirectly creates volcanoes. Terrestrial planets Interior structure of terrestrial planets How do we see inside of a planet? Interior structure of terrestrial planets Interior model based on combined information about a planet's density, seismic activity (such as frequency and intensity of earthquakes), magnetic field, heat, mountain ranges Interior structure of terrestrial planets Atmosphere of terrestrial planets Giant planets Giant planets Interior structure of giant planets Interior structure of giant planets Atmosphere of giant planets Scale height? Condensable species in giant planets ‣ Aqueous clouds (liquid water mixed with NH3 and H2S) condense deep at 300-400K ‣ Water could condense at 270K ‣ Ammonia condense at 230K through reaction of NH3+H2S- >NH4SH (ammonium hydrosulfide) ‣ Methane condense at 80K Sizes of Moons Moons of Jupiters Galilean moons in resonance Io: tidal heating Moons of Saturn:Titan From Huygens, landed on surface From Cassini Moons of Saturn:Titan ‣ Most massive moon in solar system, ~0.4 earth mass ‣ Density 1.8g/cm^3, half water and half rock ‣ Ammonia condense at 230K through reaction of NH3+H2S- >NH4SH (ammonium hydrosulfide) ‣ Only moon has significant atmosphere (N2, CH4) Nice model Nice model ‣ Late heavy bombardment (intense impact occur 600Myr based on crater record on the moon and on the terrestrial planets) ‣ Trojans (Primordial trojans escape and a new planetesimal from outer disk are captured, result in a wide range of inclination.) ‣ Outer planet’s satellite (large inclination, irregular orbits due to capture event) ‣ Kupier belt (Neptune’s migration can produce both resonant and non-resonant populations ) SI vs CGS 1. CGS: tradition; elegant equations under some circumstances, mostly adopted in astrophysics 2. SI: commonly used in all research fields SI unit vs CGS unit SI CGS asteroid, moon, pluto? High energy physics classical equations of electricity and magnetism SI CGS.
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